This invention relates to synchronizing signal separator circuits for use in television receivers.
The television broadcast signal comprises a combination of picture information (which includes both luminance and chrominance components), sound information, and synchronization signals. The latter take the form of pulse-like signals occurring at both the horizontal and vertical scan rates which are interspersed between the scan interval picture components in the period generally referred to as the retrace interval. In addition, the polarity of these scan synchronizing signals corresponds to black picture elements but their magnitude is in excess of black (i.e., "blacker than black" level). Deflection synchronizing signals also are representative of unmodulated radio frequency carrier making them particularly useful as an indication of signal strength for the receiver's automatic gain control AGC system.
Because the synchronizing signals or "sync pulses" occupy amplitude portions of the signal in excess of the picture information extremes, they are most generally separated from the remainder of the signal by an amplitude, or threshold, responsive circuit. The types of circuitry used to perform this synchronization signal separation are of almost endless variety but all may be said to perform several basic functions. First, most sync separators include a clamp circuit in which a voltage, in some measure proportional to the synchronization pulse amplitude, is stored as a reference bias. Secondly, a clipper or slicing circuit which is responsive to the clamp level conducts only during a selected portion of the sync signal to the exclusion of picture components. Finally, processing circuitry increases the amplitude of the recovered synchronization pulses and forms them into appropriate limited digital-type signals which may readily be utilized by the receiver deflection synchronization circuitry. The processing circuitry generally includes noise elimination systems the importance of which to the present invention is made more apparent below.
As mentioned, the sychronization pulse, because of its direct relationship with carrier amplitude, forms the basis for the majority of AGC system action within the receiver. In contrast, the amplitude of the picture information varies greatly due to scene content rather than signal strength alone and is therefore not a reliable indicator of required gain correction. As in the case of synchronization signal separation, the variety of AGC systems in the art is practically endless, but all perform the functions of sampling the received signal strength (i.e., sync pulse level) and producing an error voltage which is applied in a negative feed-back manner to gain control circuitry within the tuner and intermediate frequency amplifiers to provide a closed loop system having a substantially constant output signal level.
It has been found by practitioners in the art that coincidence gating the AGC system sampling circuit (often called the AGC detector) produces enhanced performance by assuring that the AGC system will respond solely to the carrier indicative sync pulses and not the varying picture components or excessive noise within the signal absence of horizontal synchronization. Most commonly, AGC coincidence gating is accomplished by providing an "AND" gate responsive to both the sync pulse signal and the horizontal deflection retrace signal which enables the AGC detector only in the event a sync pulse and retrace signal are simultaneously applied. Coincidence gating provides substantial improvement in television receiver AGC system performance. However, it does under conditions of extreme and rapid signal transition produce a malfunction commonly referred to as AGC "lock-out". This malady is well-known in the art manifesting itself in the absence of synchronization pulses when either an abrupt overload (i.e. signal excess) or signal decrease is imposed upon the receiver. In the case of an overload, the overdriven intermediate frequency amplifier cannot handle the excess signal and goes into limiting before the AGC system can respond with the needed gain reduction. This limiting raises much or all of the picture information beyond the level of sync signals creating a condition which the receiver noise processing system interprets as noise. The response of the noise processor is to turn off the sync separator. In the event of an abrupt reduction of signal or temporary signal loss, the signal excursions of sync information drop below the clamp level and no sync pulses are recovered. In both cases, the actual lock-out arises because of the AGC systems' inability to respond with an appropriate gain adjustment in the absence of synchronizing pulses to enable the coincidence gate.
A common system of preventing such AGC lock-out is set forth in U.S. Pat. No. 3,624,290. Simply stated, the system therein sets forth a capacitive coupling between the sync separator and the AGC coincidence gate in such a manner that a "hold-off" voltage is produced between sync pulses. In the absence of synchronizing pulses, the hold-off voltage at the coincidence gate established by the sync signals begins to diminish. If the absence of sync becomes prolonged (as it would during AGC lock-out), the hold-off voltage becomes insufficient to inhibit the coincidence gate which in turn overrides the coincidence requirement. Upon override of the coincidence gate the AGC system samples the applied signal during each horizontal retrace signal allowing recovery of the AGC system. This system provides a satisfactory performance solution to the AGC lock-out problem but requires a large capacitor for the AC coupling between sync separator and AGC coincidence gate. In the environment of integrated circuitry, such large capacitors are not readily formed within the integrated circuit and the use of this system necessitates using additional terminals for coupling via an external capacitor. The number of such terminals available in most integrated circuits is limited and it is desirable to avoid such situations where possible.
In addition to the aspects of synchronizing signal separation associated with AGC system performance there are, of course, criteria of effective signal separation which relate to the synchronization of the horizontal and vertical deflection systems. It is desirable in many television receivers, and perhaps mandatory in others, that the amplitude and duration of the separated sync pulses supplied to the deflection synchronizing systems be maintained substantially constant. It is desirable to recover the sync signals unaltered in duration. However, the concern is not so much the fidelus reproduction of the received synchronizing signals because predictable or "fixed" distortion of the amplitude or width, or both together, may be compensated for in synchronizing system design. The most serious concern rests with variations of sync signal in response to differing signal conditions. Such variable distortions cannot readily be compensated for and invariably cause disturbances of the deflection synchronizing system. For these reasons, it is a primary objective in the fabrication of most television receiver sync separator circuitry to provide substantially constant amplitude synchronizing signals the durations of which remain preserved.
Accordingly, it is an object of the present invention to provide an improved synchronizing signal separation circuit for use in a television receiver. It is another object of the present invention to provide a synchronizing signal separation circuit which lends itself to inclusion within an integrated circuit and requires a minimum number of external terminals. It is a further object of the present invention to provide an improved synchronizing signal separation circuit which produces synchronization pulses having substantially constant amplitude and of unaltered duration.